We recently have used a modified preplate technique to isolate 3 populations of myogenic cells from normal mouse skeletal muscle based on their adhesion characteristics and proliferation behaviors. Although 2 of these populations display satellite cell characteristics, the third population consists of long_term proliferating (LTP) cells expressing hematopoietic stem cell markers. The stem cell characteristics of the LTP cells include the abilities to retain their phenotype for more than 30 passages in culture (self-renewal) and to differentiate into various lineages, including muscle, neural, osteogenic, and endothelial. The transplantation of the LTP cells, in contrast to satellite cell transplantation, significantly improved the efficiency of muscle regeneration and dystrophin delivery to dystrophic muscle. The overall goal of our proposed project is to further investigate important features of the LTP cells (i.e., sources and mechanisms involved with the improved transplantation capacity) that are paramount to their potential use in transplantation to improve muscle regeneration and deliver dystrophin to dystrophic muscle. In this project we will investigate potential sources of the LTP cells by determining their relationship to satellite cells and to the Side Population (SP) stem cells derived from skeletal muscle (Aim #1). We subsequently will investigate mechanisms by which the LTP cells display an improved transplantation capacity in skeletal muscle. We will evaluate the relative effects of the proliferation and differentiation kinetics of the cells (Aim #2) and determine the possibility that immune_privileged cell behavior also plays a role in the increased regenerative capacity of LTP cells in skeletal muscle (Aim #3). Analysis of the relative importance of the LTP cells' self-renewal and multipotent capabilities will shed light on the contribution of stem cell characteristics to the improved transplantation capacity in skeletal muscle (Aim #4). This project might increase our understanding of the basic biology of myogenic cell populations that display stem cell characteristics. This in turn may unveil new techniques to improve muscle regeneration in dystrophic or injured skeletal muscle via the transplantation of muscle-derived stem cells.